Machine tools abstract
A controller for optimization of metal-working on CNC-operated
machine tools, includes a first unit for monitoring the torque of
the main drive of the machine tool to establish the actual, instantaneous
cutting torque, a second unit for setting the rated cutting torque
in the teaching mode in dependence on the main-drive torque as monitored,
a third unit for calculating the feed rate required to maintain
the cutting torque at a constant level and controlling the feed
drive, and a fourth unit responsive to the monitored main-drive
torque and providing feed rate limiting signals to the third unit
for protecting the tool against breakage. The unit for calculating
the feed rate is addressed by a compensator unit responsive to signals
from a comparator unit comparing the torque as set with the actual,
instantaneous torque as indicated by the first unit and to signals
from an identifier unit calculating the instantaneous cross- sectional
area of the cut in response to signals from both the first, main-drive
torque monitoring unit and the feed-rate calculating unit. A method
for optimization of metal-working on CNC-operated machine tools
is also described.
Machine tools claims
What is claimed is:
1. A system for adaptively controlling a feed rate F of a milling
cutter relative to a workpiece, the milling cutter constituting
part of a machine tool having a main drive, the system comprising:
(a) a torque monitor for monitoring an actual main drive cutting
torque M;
(b) a torque comparator for calculating .DELTA.M where .DELTA.M=M.sub.0
-M and M.sub.0 is a predetermined reference main drive cutting torque
established for the milling cutter and the workpiece material; and
(c) a feed rate controller for determining the feed rate F as a
function of .DELTA.M;
wherein
said feed rate controller includes means to calculate an instantaneous
cross-sectional area .rho. of a cut of the workpiece being worked
on by the milling cutter and determines the feed rate F as a function
of .rho. to substantially stabilized M such that .DELTA.M.fwdarw.0.
2. The system according to claim 1 wherein said feed rate controller
calculates said cross-sectional area .rho. from the general relationship
M=AF.sup.y .rho..sup..gamma. where A,y and .gamma. are coefficients
dependent on the milling cutter and the workpiece material.
3. The system according to claim 1 further comprising a spindle
speed controller for adaptively controlling the spindle speed of
the milling cutter to obtain a desired predetermined tool service
life T.sub.0.
4. The system according to claim 1 further comprising a vibration
suppression unit for minimizing vibrations of the milling cutter
below a predetermined threshold.
5. The system according to claim 1 further comprising a feed drive
current analyzer for reducing the feed rate F during stock removal
along a thin walled workpiece section.
6. A method for adaptively controlling a feed rate F of a milling
cutter relative to a workpiece, the milling cutter constituting
part of a machine tool having a main drive, the method comprising
the steps of:
(a) monitoring an actual main drive cutting torque M;
(b) calculating .DELTA.M where .DELTA.M=M.sub.0 -M and where M.sub.0
is a predetermined reference main drive cutting torque established
for the milling cutter and the workpiece material; and
(c) determining the feed rate F as a function of .DELTA.M;
wherein
step (c) includes calculating an instantaneous cross-sectional
area .rho. of a cut of the workpiece being worked on by the milling
cutter and determining the feed rate F as a function of .rho. to
substantially stabilized M such that .DELTA.M.fwdarw.0.
7. A method according to claim 6 wherein the step of determining
the feed rate includes calculating the cross-sectional area .rho.
from the general relationship M=AF.sup.y .rho..sup..gamma. where
A,y and .gamma. are coefficients dependent on them milling cutter
and the workpiece material.
8. The method according to claim 6 further comprising the step
of:
(d) adaptively controlling the spindle speed of the milling cutter
to obtain a desired predetermined tool service life T.sub.0.
9. The method according to claim 6 and further comprising the steps
of:
(e) monitoring the vibrations of the milling cutter;
(f) comparing said vibrations to a predetermined threshold;
(g) modifying the feed rate to substantially suppress said vibrations
below the predetermined threshold; and
(h) restoring the feed rate to its original value for as long as
said vibrations are below the predetermined threshold.
10. The method according to claim 6 further comprising the steps
of:
(i) monitoring the feed drive current of the milling cutter;
(j) analyzing the feed drive current for reduced harmonic levels
indicative of stock removal along a thin walled workpiece section;
and
(k) reducing the feed rate on the detection of said reduced harmonic
levels .
Machine tools description
The present invention relates to a controller and a method for
optimization of metal-working on CNC-operated machine tools, especially
on CNC-operated milling machines and machining centers.
While CNC-operated machine tools have existed for years, their
efficiency and usefulness has been limited by their incapability
to take into account many factors in the programming stage which
influence production efficiency, including: number of workpieces
in a run, operating cost, tool replacement time, tool cost, etc.
In addition, the rigidly deterministic nature of CNC-operated machine
tool programming is incapable of allowing for unforseeable changes
in real-time cutting conditions such as depth and width of metal
cutting, tool wear, non-uniformity of workpiece blank, etc.
A recent development in the field of CNC-operated machine tools
provides for apparatus for controlling a machine tool as a function
of torque load on a cutting tool when the torque load respectively
exceeds or falls below a predetermined upper or lower critical torque
load. For example, U.S. Pat. No. 4237408 describes critical torque
loads including, inter alia, a catastrophic torque limit relative
to the machine structure, a catastrophic torque limit relative to
a particular tool and a minimum torque limit that should be present
if a cutting tool is in contact with the workpiece.
It is one of the objects of the present invention to overcome the
limitations and disadvantages of today's CNC-operated machine tools
and to provide an optimizing controller for machine tools, in particular
for CNC-operated milling machines and machining centers, which calculates
the optimal cutting modes according to production efficiency criteria,
and automatically provides adaptive feed and spindle speed control
responding to real-time cutting conditions, maintains a constant
and presettable spindle torque and/or tool life, ensures optimal
machining operation, prevents tool breakage and indicates tool status.
According to the invention, this is achieved by providing a controller
for optimization of metal-working on CNC-operated machine tools,
having a main drive powering the tool spindle of said machine tools
and feed drives powering the feed mechanism of said machine tools,
said feed drives being controllable to produce a feed rate determined
either by a predetermined setting of the cutting torque produced
by said tool spindle, or by said controller overriding said setting
in a teaching mode of said controller, comprising a first unit for
monitoring the torque of the main drive of said machine tool to
establish the actual, instantaneous cutting torque; a second unit
for setting the rated cutting torque in said teaching mode in dependence
on said main-drive torque as monitored; a third unit for calculating
the feed rate required to maintain said cutting torque at a constant
level and controlling the feed drive of said machine tool; a fourth
unit responsive to said monitored main-drive torque and providing
feed rate limiting signals to said third unit for protecting the
tool against breakage, characterized in that said unit for calculating
said feed rate is addressed by a compensator unit responsive, on
the one hand, to signals from a comparator unit comparing said torque
as set with the actual, instantaneous torque as indicated by said
first unit and, on the other hand, to signals from an identifier
unit calculating the instantaneous cross-sectional area of the cut
in response to signals from both said first, main-drive torque monitoring
unit and said feed-rate calculating unit, said compensator unit
facilitating a high-precision stabilization of said torque.
The invention furthermore provides a method for optimization of
metal-working on CNC-operated machine tools having a main drive
powering the tool spindle of said machine tools and feed drives
powering the feed mechanism of said machine tools, said feed drives
being controllable to produce a feed rate determined by a predetermined
setting of the cutting torque produced by said tool spindle, or
by said controller overriding said setting in a teaching mode of
said controller, comprising the steps of monitoring the torque of
the main drive of said machine tool to establish the actual, instantaneous
cutting torque; setting the rated cutting torque in said teaching
mode in dependence on said main-drive torque as monitored; calculating,
in a feed rate calculating unit, the feed rate required to maintain
said cutting torque at a constant level and controlling the feed
rate of said machine tool; providing feed rate limiting signals
to a feed rate calculating unit for protecting the tool against
breakage; comparing, in a comparator unit, said torque as set, with
said actual, instantaneous torque; calculating, in an identifier
unit, the instantaneous cross-sectional area of the cut in response
to signals produced by both said main-drive torque monitoring unit
and said feed rate calculating unit; feeding the signals from said
two units to a compensator unit, and feeding the signals from said
compensator unit to said feed rate calculating unit, thereby achieving
high-precision stabilization of said cutting torque.
The invention will now be described in connection with certain
preferred embodiments with reference to the following illustrative
figures so that it may be more fully understood.
With specific reference now to the figures in detail, it is stressed
that the particulars shown are by way of example and for purposes
of illustrative discussion of the preferred embodiments of the present
invention only, and are presented in the cause of providing what
is believed to be the most useful and readily understood description
of the principles and conceptual aspects of the invention. In this
regard, no attempt is made to show structural details of the invention
in more detail than is necessary for a fundamental understanding
of the invention, the description taken with the drawings making
apparent to those skilled in the art how the several forms of the
invention may be embodied in practice .
In the drawings:
FIG. 1 is a block diagram of a first embodiment of the controller
according to the invention;
FIG. 2 is a diagram illustrating the effect, on the feed-rate values
and the torque values, of the compensator unit;
FIG. 3 is a block diagram of a second embodiment of the controller
according to the invention; and
FIGS. 4 and 5 illustrate a third and a fourth embodiment, respectively,
of the controller according to the invention.
The principal input parameters of the first and second embodiments
of the controller according to the present invention are one or
more of the main-drive parameters which are proportional to the
cutting torque M. The principal output parameter is a signal determining
the feed rate F as a function of M, the task fulfilled by the invention
being to maintain this torque at a steady level determined in dependence
on the properties of the specific milling cutter used. The required
values can be found in appropriate tables.
Another concept of the present invention is the teaching mode in
which, instead of the maximum rated cutting torque M.sub.o, a maximum
torque M.sub.o ' is determined during the machining of one or several
of the first identical workpieces. The teaching mode is particularly
effective for large runs of identical workpieces.
Another important parameter used by the controller according to
the invention is .rho.[mm.sup.2 ], designating the cross-sectional
area of the cut (for short, area of cut), which is the product of
the cut width (b) and cut depth (h).
Referring now to the drawings, there is seen in FIG. 1 a block
diagram of a first embodiment of the controller according to the
invention, comprising a housing 2 attachable to a CNC-operated milling
machine and accommodating the various units of the controller, and
a panel 4 which is accessible to the operator.
On the panel 4 is located a switch 6 for selecting: initiation
of the Teaching Mode (TM) ("initiate"); "Run"
for M.sub.o settings determined in the teaching mode, and operation
with predetermined M.sub.o settings ("without TM"). In
the latter, the value for M.sub.o is set on the selector 8. Other
elements on panel 4 include a starting button 10 and a tool status
indicator 12 which lights up, or provides, e.g., an acoustic warning,
when the tool is worn beyond a certain limit.
There is seen a monitoring unit 14 in which the instantaneous main-drive
cutting torque M (as applied by the milling cutter) is monitored.
The signal M from the monitoring unit 14 is fed to a number of
other units of the controller:
a) the unit 16 for setting the rated cutting torque M.sub.o for
application in the teaching mode;
b) a tool protection unit 18 which supplies feed rate limiting
signals to a feed rate calculator 20;
c) a unit 22 for identifying the instantaneous value of .rho.,
also addressed by the signal from the feed rate calculator 20 and
d) a comparator unit 24 which compares the set torque M.sub.o with
the actual, instantaneous torque M.
According to the position of the mode switch 6 a logic element
26 provides the comparator unit 24 with the M.sub.o value as determined
either by unit 16 or by the manual selector 8.
The controller also includes a self-diagnostic unit 28 interposed
between the start button 10 on the panel 4 and the feed rate calculator
20. When the button 10 is pressed, the unit 28 performs a test of
the entire system and, if the latter is found operational, provides
an enabling signal to the feed rate calculator 20.
The heart of the controller is constituted by a compensator unit
30 in cooperation with the already-mentioned .rho.-identifier unit
22.
The following is an explanation of the considerations underlying
the compensation principle.
The feed rate is determined by the difference .DELTA.M between
the set value M.sub.o or M.sub.o ' and the actual value M.
The metal-cutting process (as static process) can be represented
by the expression:
where:
.rho.=the already-mentioned area of cut;
F=feed rate, and
A, y, .gamma.=coefficients depending on tool type and metal-working
conditions.
Seeing .DELTA.M as the error of cutting torque stabilization, it
can be defined as: ##EQU1## where: K.sub.c =CNC gain (static), and
K.sub.1 =current monitor gain.
However, in real-life machining, .rho.<<1/K.sub.1 K.sub.c
A, as a result of which .DELTA.M.apprxeq.M.sub.o, or M.apprxeq.O,
making it impossible to achieve cutting torque stabilization with
medium and small .rho.-values.
In order to secure for M independence from changes of .rho., it
is necessary to provide a compensator unit with variable gain K.sub.k
: ##EQU2## with B being a constant.
To calculate K.sub.k it is thus necessary to determine .rho. at
every instant throughout the cutting process, which is done by unit
22 according to the assumption that .rho. is proportional to the
ratio .DELTA.M/F.sup..varies., where .varies. is determined for
each material to be cut.
The effect of the compensator unit is shown in FIG. 2 in which
the solid curves 32 and 34 indicate the values of F and M/M.sub.o
as functions of .rho. (specifically, of the cut height h) with compensation,
and the dashed curves 36 and 38 indicate the same values F and M/M.sub.o
without compensation.
The feed rate of the machine tool is obviously controlled by the
output F of the feed rate calculator 20.
FIG. 3 shows another embodiment of the controller according to
the invention. This embodiment differs from the previous embodiment
in that the controller is inaccessible to the operator, being addressed
only by the CNC program. Added elements in this embodiment are a
program interface 40 linking the controller to the CNC program and
a memory unit 42 for the rated torque M.sub.o of a number of different
tools N (as marked MN.sub.3 -MN.sub.25) to be used in the machining
process, with MN.sub.0 and MN.sub.1 signifying selection of the
teaching mode and MN.sub. --without teaching mode. The rest of the
unit is identical with the units of the previous embodiment and
operate in the same manner.
The embodiment illustrated in the block diagram of FIG. 4 is intended
for the optimization of machining operation on the basis of either
one or the other of two criteria:
1) maximum metal removal per unit time (mm.sup.3 /min);
2) minimum cost of removal of unit volume of metal ($/min).
It is possible to select a compromise between these criteria.
The embodiment of FIG. 4 comprises all the units described in connection
with FIGS. 1 and 3 (except for the panel 4 and its elements), as
well as some additional units to be described further below.
While the first criterion is taken care of by the "F-loop"
comprised of units 20 22 24 and 30 (FIGS. 1 and 3) and is conditional
upon M=M.sub.o, the second criterion requires the introduction of
an additional unit, 44 which constitutes the operative part of
an "S-loop", inasmuch as it is meant to control the speed
IS) of the tool spindle. This unit consists of a calculator 44
which realizes the expression: ##EQU3## where: A.sub.3 =coefficient
dependent on the specific tool used;
.varies..sub.3 .varies..sub.4 .varies..sub.5 =coefficients depending
on the material machined;
.rho.32 area of cut, supplied by the identifier unit 22
F=feed rate, and
T.sub.o =tool service life required for selected optimization criteria.
The first criterion is conditional upon the relationship: ##EQU4##
The second criterion is conditional upon the relationship: ##EQU5##
where: m=coefficient depending on the specific tool used and material
machined;
.tau.=auxiliary or idle time (min);
D=cost of tool ($);
B=cost of machining per min ($/min).
The calculator 44 has five inputs:
a) coefficients A.sub.3 for the tools N3-N25 (from memory 46 addressed
by input MN.sub.3 -MN.sub.25); p1 b) coefficients .varies..sub.3
.varies..sub.4 .varies..sub.5 for four different groups of materials
(from memory 48 addressed by input MN26-MN28);
c) signal F (from calculator unit 20);
d) area of cut .rho. (from the identifier unit 22), and
e) projected tool service life T.sub.o (from unit for calculation
of T.sub.o).
Input MN.sub.o initiates the teaching mode and input MN.sub.1 runs
the teaching mode for all tool diameters.
The outputs of the controller of this embodiment are the same as
with the previous embodiment (tool status and feed rate control
signal F), with the addition of the speed control signal S.
The embodiment represented in FIG. 5 has all the features described
in the previous three embodiments, with the addition of two further
features, namely, a circuit suppressing machine tool vibrations
and chatter, and a circuit facilitating the finish machining, at
high precision, of thin wall sections of workpieces.
The first of these features comprises a vibration analyzer 50 addressed
by any suitable transducer 51 responding to vibrations and chatter
of the machine. The output of the transducer 51 is analyzed by unit
50 which produces a signal fed to the feed rate calculator 20 which,
in response, modifies the feed rate F to the degree required to
suppress the vibrations, returning it to the original rate once
this has been achieved.
The problem with thin sections is their elastic deformability under
the cutting pressure of the milling cutter. Thus milling an aluminum
wall of a thickness of, e.g., 2.5 mm and a length of 200 mm, taking
a cut of a depth of 0.5 mm at a feed rate of 500 mm/min, a cutter
speed of 1000 rpm and a tool diameter of 12 mm, will produce an
error of 0.04 mm, while milling a section of a thickness of 10 mm
at identical cut depth, feed rate, speed and tool will produce an
error of only 0.005 mm. This difference is, of course, due to the
"giving in", and subsequent spring-back, of the thin section,
necessitating a reduction of the feed rate when the milling cutter
arrives at such a thin section.
This not only complicates the CNC-program, but it is also difficult
to determine at which point, after a heavy section, the thin section
effectively begins. Also, a worn cutter will increase the deforming
force which, with a new cutter, would be much smaller.
It is the task of the present embodiment to automatically reduce
the feed rate the moment wall deformation is detected.
It was found that certain harmonics of the feed-drive current are
reduced during the milling of thin walls, due to the change of frequency
characteristics of the electrical-mechanical loop of which the thin
section is a part. Thus, based on a dispersive analysis of feed-drive
current signals, it is possible to form special signals indicating
the effective beginning and ending of a thin section. These signals
are used to reduce the feed rate during the machining of such thin
sections, thus increasing the accuracy of the machining operation.
The added circuit of the embodiment of FIG. 5 comprises a suitable
sensor 52 responsive to the feed-drive current, feeding an analyzer
54 for analyzing the harmonics of the feed-drive current, which
analyzer addresses a signal transducer 56 producing signals that,
fed to the feed rate calculator 20 modify the output signal of
the latter, reducing the feed rate whenever the sensor 52 and analyzer
54 indicate the effective beginning of a thin section, and restoring
the previous feed rate when the sensor 52 and analyzer 54 indicate
the ending of this section. The embodiment of FIG. 3 is particularly
suitable for CNC- operated machining centers using a pre-programmed
sequence of different tools, and is more efficient than the previous
embodiment, particularly due to the provision, as shown in FIG.
3 of the memory unit 42 which eliminates the need to reset the
controller each time a tool is changed.
It will be evident to those skilled in the art that the invention
is not limited to the details of the foregoing illustrated embodiments
and that the present invention may be embodied in other specific
forms without departing from the spirit or essential attributes
thereof. The present embodiments are therefore to be considered
in all respects as illustrative and not restrictive, the scope of
the invention being indicated by the appended claims rather than
by the foregoing description, and all changes which come within
the meaning and range of equivalency of the claims are therefore
intended to be embraced therein. |